No it does not have to be all the same material but good if it is.
I love to do scrappy's and just take pieces and start sewing in all directions. I've done about 5 tops so far and now venturing into sewing scraps and using them in rag quilts. As a matter of fact I'm doing one now and it is turning out very nice.
I also went through my trash bags full of scraps yesterday and I'm going to sew them together and than cut them to size to make fabric goodie bags, pillows and more.
I've also been accumalating colorful fabric pieces to make a crazy quilt or some such crazy thingie one day.